The present invention relates to an information recording material suitable for use in an optical disk system. More particularly, the present invention is concerned with an information recording material which is excellent in carrier-to-noise ratio, storage stability and archivability.
Information storage systems in which information is stored by selective ablation, evaporation, removal or modification of a recording medium irradiated spot-wise with a focused laser beam or the like having high density or strong energy are known as the so-called heat mode recording process to those skilled in the art. The heat mode recording process is a dry process which does not any treatment with chemicals or other treating solutions and in which a real-time recording can be made. According to this process, information can be quickly recorded in a high contrast form with a large capacity in respect to the amount of information recordable per unit area of the recording medium, and additional information can be recorded later. Due to such advantages, the heat mode recording process has a wide variety of applications in the field where a micro-image recording medium, a computer-output microfilm, a video disk, a computer signal memory medium or the like is involved.
The heat mode recording medium which is used particularly as the video disk, the document filing and the computer signal memory medium generally comprises an information recording layer formed on a suitable substrate such as a circular plate or disk of glass or a synthetic resin. When a laser beam modulated (frequency: several hundreds KHz to several tens MHz) is focused on the recording layer rotated at a high speed, pits (holes) of 1.5.mu. or less in width are formed in the areas where the recording layer is irradiated with the laser beam, whereby information is recorded in the recording medium. The positions and sizes of the formed pits depend on the output wave form of laser beam modulated and correspond to the input information. The readout of the information stored in the recording medium is carried out by applying to the recording medium being rotated at a high speed a focused weak laser beam whose intensity is lower than the threshold value to produce a pit in the recording layer, and electrically detecting the variation of the reflected light by means of a photoelectric transducer, followed by demodulation of the obtained signals by means of a decoder.
As a recording material to be used in the above-mentioned heat-mode recording process, various materials have hitherto been proposed. However, none of them are satisfactory with respect to sensitivity, carrier-to-noise ratio, (hereinafter referred to as "C/N ratio") storage stability, archivability and the like.
The present inventors have previously conducted extensive research with a view to eliminating the drawbacks accompanying the conventional heat mode recording materials and succeeded in attaining improvements to some extent both in sensitivity and C/N ratio by providing at least one mixture layer of a metallic compound and a metal on at least one side of the metallic recording layer (see European patent application Publication No. 0045183). The recording material of the structure as mentioned just above exhibits an excellent sensitivity as well as a relatively high C/N ratio especially when recording is effected by applying a laser beam through a transparent substrate. In the case where recording is effected by semiconductor laser beam irradiation, it is required that the recording material to be employed have a high C/N ratio and a high sensitivity. In this respect, the information recording material of the type as mentioned above which is relatively excellent in both C/N ratio and sensitivity can advantageously be employed in recording by means of a semiconductor laser. Generally, in the digital recording system utilized for data or document filing, a recording medium or a material to be used in such system is required to have a C/N ratio of 40 dB or more. When recording by means of a semiconductor laser beam is effected using the recording material disclosed in the above-mentioned European patent application Publication No. 0045183, it is possible to obtain a C/N ratio as high as approximately 47 dB at a standard information recording frequency of 1 MHz (pulse width: about 500 nsec). As is apparant from the above, the above-mentioned recording material which has at least one mixture layer of a metallic compound and a metal on at least one side of the metallic recording layer can perform a digital recording by means of a semiconductor laser beam effectively, and, therefore, is excellent from a practical viewpoint.
However, in the case of video image signal memory or still video recording where a pulse-width modulation system or the like is employed for the information recording and for the readout of the recorded information, the use of a recording material which can exhibit a C/N ratio higher than 47 dB is required. As a recording material having a C/N ratio higher than 47 dB, a recording material comprising a thin film of a chalcogen compound or that comprising a thin film of a suboxide of Te or the like have hitherto been proposed. However, such recording materials are still insufficient for satisfying the demand for a recording material not only excellent in recording characteristics but also in storage stability and archivability.
Accordingly, under such circumstances, there is a strong demand in the art to develop a recording material which is not only excellent in recording characteristics but also in storage stability and archivability.